This invention relates to a semiconductor device and, more particularly, to a semiconductor saturable absorber device for use in mode-locked lasers for the generation of short and ultrashort optical pulses. The invention also relates to a mode-locked laser comprising a semiconductor saturable absorber device.
Lasers emitting short or ultrashort pulsesxe2x80x94i.e. pulses in the picosecond and in the sub-picosecond rangexe2x80x94are known in the art. A well-known technique for short or ultrashort pulse generation is mode locking. Mode locking is a coherent superposition of longitudinal laser-cavity modes, It is forced by a temporal loss modulation which reduces the intracavity losses for a pulse within each cavity-roundtrip time. This results in an open net gain window, in which pulses only experience gain if they pass the modulator at a given time. The loss modulation can be formed either actively or passively. Active mode locking is achieved, for instance, using an acousto-optic modulator as an intracavity element, which is synchronized to the cavity-roundtrip time. However, for ultra-short-pulse generation, passive mode-locking techniques are preferred, because only a passive shutter is fast enough to shape and stabilize ultrashort pulses. One option to implement passive mode locking is to rely on a saturable absorber mechanism, which produces decreasing loss with increasing optical intensity. When the saturable-absorber parameters are correctly adjusted for the laser system, stable and self-starting mode locking is obtained.
A broad class of semiconductor saturable absorbers are known in the state of the art. Such saturable absorbers usually comprise a layered structure with one or several layers of a semiconductor material having a non-linear absorption characteristics at the laser frequency. By choosing appropriate layers with specifically prepared surfaces, a large variety of different optical properties can be achieved for such structures. Especially, these structures may be designed to be anti-resonant or resonant, and they may have a high Q-factor or a low Q-factor. For mode-locking in lasers, antiresonant devices have been the structures of choice. This is because resonant structures have much narrower tolerances of growth accuracy and the high field intensities in the structures lead to high losses and a delicate dependence of the characteristics of the entire laser system on the absorber device properties such as its quality etc. of the structure. Known structures of this kind include the antiresonant Fabry-Perot Saturable Absorbers (A-FPSAs), and conventional semiconductor saturable absorber devices (cf. U. Keller et al., xe2x80x9cSemiconductor saturable absorber mirrors for femtosecond to nanosecond pulse generation in solid-state lasersxe2x80x9d, Journal of Selected Topics in Quantum Electronics (JSTQE), Vol. 2, No. 3, 435-453, 1996, incorporated herein by reference) being low Q antiresonant devices. Further examples of structures include the Saturable Bragg Reflector (SBR). Recently, a low-field enhancement (i.e. low Q) but resonant saturable absorber device (LOFERS) design has been invented. A corresponding U.S. patent application by Weingarten, Spxc3xchler, Keller, and Krainer is pending and has been attributed application Ser. No. 10/016,530.
Jung et. Al. have disclosed a xe2x80x9cthin-absorberxe2x80x9d device for operating around 840 nm which uses a layer of dielectric to complete the semiconductor structure. This layer is described as an effective xe2x80x9canti-reflectionxe2x80x9d (AR) coating. (Electronics Letters Feb. 1995, pp. 288-289).
For many applications, InGaAs is a preferred absorber material due to its inherent properties. This, however, brings about new challenges when applied to lasers designed for communication technology purposes. In communication technology, lasers operating at frequencies corresponding to a free space wavelength of 1.55 xcexcm are increasingly important. One of the challenges with 1.55 micron operation of semiconductor saturable absorber devices is the high concentration of In in the InGaAs absorber layer, required to achieve absorption at this wavelength. InGaAs is the material of choice in many known saturable absorbers. However, in order to cause the absorption edge to be energetically as low as 1.55 xcexcm, the concentration of the In replacing the Ga when going from GaAs to InGaAs has to be rather high, i.e. the absorber material is InxGa1xe2x88x92xAs with xxcx9c50%-58%, and GaAs or AlAs. The admixture of In in such a high concentration, of course, also changes other material properties than the bandgap, one of them being the lattice constant. As a consequence, a much higher lattice mismatch has to be dealt with in 1550 nm semi conductor saturable absorber devices than, for example, in 1060 nm semiconductor saturable absorber devices. For example, the natural, relaxed lattice constant of In0.53Ga0.47As is about 0.583 nm vs. 0.565 nm for GaAs and 0.566 nm for AlAs. InGaAs absorber layers grown onto or in GaAs or AlAs layers thus tend to relaxxe2x80x94i.e. to re-adopt the natural InGaAs lattice constant at the price of a certain, high amount of generated defectsxe2x80x94if a certain critical layer thickness is exceeded. The defects substantially reduce the device quality in terms of losses. Due to this lattice mismatch, most InxGa1xe2x88x92xAs (x greater than 0.5) absorber layers will relax within 1-2 nm thickness, resulting in many defects. This brings about a decrease in the optical quality of the crystalline layers grown following the absorber layer, since defects tend to propagate through layers grown by epitaxy subsequently to the relaxed absorber layers.
A further important frequency used for telecommunication purposes corresponds to the free space wavelength of essentially 1.3 xcexcm (i.e. the frequency equals the speed of light divided by about 1.3 xcexcm.). In this case, the In concentration in the absorber material is lower, i.e. xxcx9c0.4. Although the lattice mismatch between InxGa1xe2x88x92xAs (xxcx9c0.4) and GaAs or AlAs is not as high as for 1.55 xcexcm, ensuring epitaxial growth is still an important issue also in this system.
One method to avoid this is to grow lattice-matched layers. However, this requires using Bragg reflector materials such as InP/InGaAsP, or InP/AlGaInAs or AlInAs/AlGaInAs. The disadvantage of these material systems include a reduced index contrast between the mirror pairs, resulting in less reflectivity and less mirror reflectivity bandwidth for a given number of layer pairs (compared to GaAs/AlAs for example), more complex epitaxial growth processes in MOCVD or MBE machines, and increased losses in the structures, increased demands on the growth accuracy.
In U.S. Pat. No. 5,701,327, a semiconductor saturable absorber device for lasers operating at 1.55 micron is disclosed, which comprises a GaAs/AlAs Bragg mirror, onto which a InP xe2x80x9cstrain reliefxe2x80x9d layer is grown. The absorber layers are embedded in this InP strain relief layer. With InP grown on GaAs or AlAs, many defects are formed which may serve as recombination centers leading to an ultra-fast device response. However, such a semiconductor saturable absorber devices brings about comparably high losses.
A further challenge to be met with mode-locked solid-state lasers are Q-switching instabilities which are present at high frequencies. For passively mode-locked lasers using semiconductor saturable absorber mirror devices or similar devices for mode-locking, the onset of Q-switching instabilities limits the repetition rate (see U. Keller et al., xe2x80x9cSemiconductor saturable absorber mirrors for femtosecond to nanosecond pulse generation in solid-state lasers,xe2x80x9d Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435-453, 1996; and U. Keller, xe2x80x9cUltrafast all-solid-state laser technologyxe2x80x9d, Applied Physics. B, vol. 58, pp. 347-363, 1994).
When the conditions necessary to avoid the Q-switching instabilities in passively mode-locked lasers are examined more carefully, the following stability condition can be derived:
(Flaser/Fsat,laser)xc2x7(Fabs/Fsat,abs) greater than xcex94R xe2x80x83xe2x80x83(1)
where Flaser is the fluence in the laser material, Fsat,laser=h"ugr"/"sgr"laser is the saturation fluence of the laser material, h is Planck""s constant, "ugr" is the center laser frequency, "sgr"laser is the laser cross-section parameter (see W. Koechner, Solid-State Laser Engineering, 4th Edition, Springer-Verlag New York, 1996), Fabs is the fluence on the absorber device, Fsat,abs=h"ugr"/"sgr"abs-eff is the effective saturation fluence of the absorber, where "sgr"abs-eff is the effective cross-section parameter of the absorber device, and xcex94R is the modulation depth of the absorber device.
For further clarity we simplify Eq. (1) to the following:
xe2x80x83Slaserxc2x7Sabs greater than xcex94Rxe2x80x83xe2x80x83(2)
where Slaser is the fluence ratio in the laser material, and Sabs is the fluence ratio on the absorber. This reduced notation allows us to simplify the further discussion. To achieve the maximum figure of merit, one can change the laser design to increase the fluence ratio Slaser in the laser material, or to increase the fluence ratio Sabs in the absorber. In this document, approaches concerning the fluence ratio Sabs in the absorber and the modulation depth xcex94R are discussed.
Equation (2) can be used to scale a laser for operation at higher repetition rates. If all else remains constant (i.e., mode size in laser material and on the absorber, average power, and pulsewidth), as the repetition rate increases, the left-hand term decreases quadratically due to decreasing pulse energy. It is possible to avoid Q-switching under this condition by arbitrarily decreasing the modulation depth xcex94R. However, below a certain modulation depth, the absorber will not have a strong enough effect to start and sustain mode-locking.
The above mentioned U.S. patent application Ser. No. 10/016,530 shows an approach for solving this problem. The low field enhancement resonant design taught in this patent application leads to a field intensity which at the position of the absorber is enhanced by factors of up to 10 or more compared to conventional semiconductor saturable absorber device designs. As a consequence, the modulation depth is increased and the saturation fluence of the absorber is decreased by the same factor, if all other parameters are kept constant. By reducing the absorber thickness compared to conventional semiconductor saturable absorber devices, the modulation depth may be re-reduced to a xe2x80x98conventionalxe2x80x99 value. By this measure, the above stability condition is satisfied for pulses with reduced fluence at the absorber, i.e. for pulse trains with increased repetition frequencies.
However, this important advantage comes at the cost of other more delicate optical properties. The group delay dispersion has a much more pronounced frequency dependence compared to conventional, anti-resonant designs. The bandwidth of the device is therefore reduced.
It is an object of the invention to provide an absorber device which is useful for operation at telecommunication frequencies and which does not suffer from drawbacks of prior art absorber devices. The device should allow for high repetition rate operation. Preferably, it also works in lasers with high average output power. According to a preferred embodiment, it should therefore for example have a good thermal response, i.e. heat should efficiently be conducted away from the absorber layer. According to further preferred embodiments, the number of crystal growth defects is minimized or optimized. The invention should preferably allow for a control of the group delay dispersion and have a reasonably high bandwidth.
According to a first embodiment of the invention, a semiconductor saturable absorber mirror device for reflecting at least a proportion of electromagnetic radiation of essentially one given optical frequency impinging on said device, comprises a substrate with a Bragg reflector, and on top of this Bragg reflector a layered structure with at least one layer with saturably absorbing semiconductor material. A low index dielectric coating layer is placed on an outermost surface of said structure, the index of refraction of said dielectric coating layer being lower than the lower of said first index of refraction and said second index of refraction. The Bragg reflector and the layered structure are designed in a manner that the intensity of radiation of essentially the given optical frequency impinging on the device takes up a maximum at or near the interface between said structure and said dielectric coating layer.
The intensity takes up a maximum xe2x80x9cat or nearxe2x80x9d the interface, in the context of this specification, means that the intensity is not substantially reduced at the position of the interface compared to the closest intensity maximum. For example, this is the case within a distance from the intensity maximum of about a tenth or a sixteenth of the wavelength of the radiation in the respective material.
The thickness of the coating layer may be between a few nanometers and half a wavelength of radiation of the given optical frequency in the dielectric material. It may, as an alternative also be greater than half a wavelength. According to a first special embodiment it corresponds to substantially (2n+1)/4 times the wavelength of said radiation in said dielectric layer, where nxe2x89xa70=any whole number, so that the intensity at the device surface is at a minimum, the device thus being substantially anti-resonant. According to a second special embodiment it is at least an eighth of a wavelength but less than a quarter wavelength. According to yet another special embodiment it is more than a quarter wavelength but less than three quarters of a wavelength.
The index of refraction of the dielectric coating layer should be lower than the layers of the structure and of the Bragg mirror. For example, the index of refraction is below 2.2 or even below 2 or below 1.8. According to particularly preferred embodiments, the index of refraction is 1.6 or lower.
According to a further embodiment of the invention, the semiconductor saturable absorber device comprises
a structure of layers, said structure comprising a semiconductor layer having a nonlinear optical absorption substantially at said frequency,
and a dielectric coating layer placed adjacent to said structure,
said dielectric coating layer having an index of refraction which is substantially lower than the index of refraction of the layers of said structure
said structure being designed and arranged such that said semiconductor layer having a nonlinear optical absorption is placed at the interface to said dielectric coating layer and the field intensity of radiation of said given optical frequency in the device has a peak at said interface.
According to another embodiment of the invention, a semiconductor saturable absorber mirror device for reflecting at least a proportion of electromagnetic radiation of essentially one given optical frequency impinging on said device is provided, comprising
a substrate,
a stack of layers placed on said substrate, said stack comprising alternately layers of a first material having first index of refraction and of layers of a second material having a second index of refraction, said first and said second indices of refraction being different from each other, said stack being designed in manner that a Bragg reflector is formed,
a layered structure comprising at least one layer with semiconductor material having a nonlinear optical absorption substantially at said frequency,
and a dielectric coating layer placed on said outermost surface of said structure, the index of refraction of said dielectric coating layer being lower than the lower of said first index of refraction and said second index of refraction,
wherein said stack of layers, said layered structure and said dielectric coating layer are designed in a manner that the field intensity of radiation of said given frequency takes up a minimum at or near the surface of said dielectric coating layer.
According to yet another embodiment, a semiconductor saturable absorber mirror device for reflecting at least a proportion of electromagnetic radiation of essentially one given optical frequency xcexd=c/xcex with 1525 nm less than xcex less than 1575 or with 1300 nm less than xcex less than 1350 impinging on said device is provided, the device comprising
a substrate,
a stack of layers placed on said substrate, said stack comprising alternately layers of a first material having first index of refraction and of layers of a second material having a second index of refraction, said first and said second indices of refraction being different from each other, said stack being designed in manner that a Bragg reflector is formed,
a layered structure comprising at least one layer with semiconductor material having a nonlinear optical absorption substantially at said frequency,
and a dielectric coating layer placed on said outermost surface of said structure, the index of refraction of said dielectric coating layer being lower than the lower of said first index of refraction and said second index of refraction,
wherein said stack of layers, said layered structure and said dielectric coating layer are designed in a manner that the field intensity of radiation of said given frequency at a position of said semiconductor material having a nonlinear optical absorption is enhanced compared to a device which comprises said substrate, said stack of quarter-wave layers, and said layered structure where the dielectric coating layer is replaced by semiconductor material with a corresponding optical thickness substantially at said frequency.
According to yet a further embodiment, a semiconductor saturable absorber mirror device for reflecting at least a proportion of electromagnetic radiation of essentially one given optical frequency impinging on said device comprises
a substrate,
a stack of layers placed on said substrate, said stack comprising alternately layers of a first material having first index of refraction and of layers of a second material having a second index of refraction, said first and said second indices of refraction being different from each other, said stack being designed in manner that a Bragg reflector is formed,
a layered structure comprising at least one layer with semiconductor material having a nonlinear optical absorption substantially at said frequency,
and a dielectric coating layer placed on said outermost surface of said structure, the index of refraction of said dielectric coating layer being lower than the lower of said first index of refraction and said second index of refraction,
wherein said layered structure comprises a spacer layer, and further comprises said layer with semiconductor material having a nonlinear optical absorption substantially at said frequency, placed adjacent to said spacer layer, and a cap layer of semiconductor material placed adjacent to said layer with semiconductor material having a nonlinear optical absorption, wherein said dielectric coating layer is placed adjacent to said cap layer, wherein said cap layer has a thickness of substantially 20 nm or less, and wherein said field intensity of radiation of said given frequency takes up a maximum substantially at the position of said layer with semiconductor material having a nonlinear optical absorption substantially at said frequency.
The Bragg reflector in any of the above embodiments may for example be a stack of quarter wave layers, for example of alternately GaAs and AlAs. As an alternative, it may be a stack of alternately xe2x85x9c wave and xe2x85x9 wave layers, or a stack forming a chirped Bragg mirror. An absorber layer may for example be InGaAs. According to one example, it may be grown directly on top of the Bragg reflector, the layered structure then consisting of an absorber layer only.
The device according to the invention features the following advantages:
Field enhancement: The structure may be substantially anti-resonant (i.e. the field intensity at the surface may be minimal), but due to the low index of refraction of the dielectric coating layer, the field inside the structure and especially at the place of the absorber layer(s) is not as strongly reduced compared to the free space intensity as in conventional semiconductor saturable absorber devices. The quotient of the field intensity at the place of the absorber layer of a device according to the invention with the corresponding field intensity in a conventional semiconductor saturable absorber device is named xe2x80x9cenhancement factor Nxe2x80x9d in the context of this application. As a consequence of the stronger field at the place of the absorber layer(s), the modulation depth is increased, and the effective saturation fluence of the absorber is reduced.
Reduced tendency of defects to propagate: Defects in the epitaxial layers will not be contiguous with defects in the coating layer and vice versa. Therefore, defects will not propagate between layers or interact with each other at the surface. Also, defects in wide-gap dielectrics are unlikely to be optically active in the near infrared and are therefore less of a problem for lasers or the kind of the laser according to the invention.
New design degrees of freedom: A direct consequence of the above mentioned enhancement of the modulation depth is that the absorber layer(s) may be grown thinner than in state of the art semiconductor saturable absorber devices for comparable laser devices. By this, the modulation depth is reduced while keeping the saturation fluence constant. In such a case, the above mentioned Q-switching threshold is improved, thus making operation at increased repetition frequencies and/or at increased average power possible. For example a thinner InGaAs absorber layer may also be grown strained and not relaxed. By this, the defect density in the absorber layer is strongly reduced. It is possible to decrease the absorber thickness by the enhancement factor N and still maintain a device which has similar performance parameters for passive mode-locking, i.e. similar mode locking driving force but reduced tendency to QML.
Passivation: The dielectric coating layer may stabilize the surface and prevent chemical reactions from taking place. Thus, it is also possible to use the field enhancement resonant structure as disclosed in the U.S. patent application Ser. No. 10/016,530, which could have a chemically unstable surfacexe2x80x94by, for example, having the absorber layer on top of the structurexe2x80x94and add said dielectric coating layer, resulting in an inert structure.
Reduction of undesired effects: As well as preventing reaction, coating layers may also reduce other effects such as emission/sublimation of As and Ga. Also, in the particular case of reaction with oxygen, GaAs and InGaAs are likely to have a very high spin-orbit coupling efficiency. When radiation impinges on the device, this can result in the photoinduced formation of a singlet-state molecular-oxygen at the surface. This is a highly reactive species. Using a dielectric layer will reduce this effect and in the case of an oxide dielectric make it irrelevant.
Compact and easy to manufacture design: The entire device may be fabricated in a simple two-step process.
The invention also concerns any laser device comprising any embodiment of the absorber device according to the invention.